Electrically driven oxygen separators are used to separate oxygen from oxygen containing feed, for example, air. Additionally, such devices are also used in purification application where it is desired to purify an oxygen containing feed by separating oxygen from the feed. Electrically driven oxygen separators can utilize tubular membrane elements having a layered structure containing an electrolyte layer capable of transporting oxygen ions when subjected to an elevated temperature, cathode and anode electrode layers located at opposite surfaces of the electrolyte layer and current collector layers to supply an electrical current to the cathode and anode electrode layers.
When the tubular membrane elements are subjected to the elevated temperature, the oxygen contained in a feed will ionize on one surface of the electrolyte layer, adjacent the cathode electrode layer by gaining electrons from an applied electrical potential. Under the impetus of the applied electrical potential, the resulting oxygen ions will be transported through the electrolyte layer to the opposite side, adjacent the anode layer and recombine into elemental oxygen.
The tubular membrane elements are housed in an electrically heated containment to heat the tubular membrane elements to an operational temperature at which oxygen ions will be transported. Additionally, such tubular membrane elements can be manifolded together such that the oxygen containing feed is passed into the heated containment and the separated oxygen is withdrawn from the tubular membrane elements through a manifold. In certain purification applications, the oxygen containing feed can be passed through the interior of the tubular membrane elements and the separated oxygen can be withdrawn from the containment.
Typical materials that are used to form the electrolyte layer are yttrium or scandium stabilized zirconia and gadolinium doped ceria. The electrode layers can be made of mixtures of the electrolyte material and a conductive metal, a metal alloy or oxide such as an electrically conductive perovskite. Current collectors in the art have been formed of conductive metals and metal alloys, such as silver as well as mixtures of such metals and metallic oxides.
In order to apply the electrical potential to the tubular membrane elements, conductors can be attached to the current collector layers. Such conductors are attached at single locations to connect the tubular membrane elements in a series or parallel electrical connection. The problem with this is that the electrical current is unevenly distributed throughout the length of each of tubular elements resulting in hot spots developing at the connection of the conductors to the tubular membrane elements. Such hot spots can lead to failure of the tubular elements. Ideally the current is distributed evenly along the length of the current collector resulting in an even temperature distribution and localized oxygen ion flow along the length of the membrane. Since the distribution of the electrical current is uneven, ionic conduction of the oxygen ions through the electrolyte layer is also uneven in that it occurs, to a large extent, at the connection of the conductors to the current collection layers.
A yet further problem is that the tubular membrane elements project through insulators and/or the heated containment that can also be insulated. Thus, at the projecting ends of the tubular membrane elements, a temperature is produced that is about 500° C. less than the temperature of the tubular elements within the heated containment that can be about 700° C. At these temperature transition zones it has been found that the electrolyte layer can undergo a chemical reduction in which the electrode chemically reduces into an electronic conductor leading to another point at which the tubular membrane elements will fail over time.
As will be discussed, the present invention provides an oxygen separation assembly that utilizes one or more tubular membrane elements and a related method in which, among other advantages, the current is more evenly distributed along the length of the tubular membrane elements as compared with prior art. Further each of the tubular elements can be modified to resist failure in the temperature transition zone as discussed above. Furthermore, the ends of the tubular membrane elements can be sealed with a plug-like member in a cost effective manner.